Powdery, cross-linked polymers which absorb aqueous liquids and blood, method for the production thereof and their use

ABSTRACT

The invention relates to crosslinked polymerizates which are capable of absorbing, which are based on partially neutralized, monoethylenically unsaturated monomers that carry acidic groups, which exhibit improved properties, in particular, with regard to their ability to transport liquids when in a swollen state, and which are subsequently crosslinked on the surface thereof at temperatures ≧150° C. with a combination consisting of polyol used as a subsequent crosslinker compound and of a cation provided in the form of an aqueous solution.

The invention relates to powdered, crosslinked polymers (superabsorbers)which absorb water, aqueous liquids, as well as blood, and have improvedproperties, particularly improved retention and improved liquidretention capability under pressure and an improved capability ofconveying liquids, and to their production and their use as absorbentsin hygiene articles and in technical fields.

Superabsorbers are water-insoluble, crosslinked polymers capable ofabsorbing large amounts of aqueous liquids and body fluids such as urineor blood with swelling and formation of hydrogels, and retaining themunder a specific pressure. As a result of these characteristicproperties, these polymers are predominantly used for incorporation insanitary articles, e.g., in diapers for babies, incontinence articles,or in liners.

Essentially, the superabsorbers commercially available at present arecrosslinked polyacrylic acids or crosslinked starch/acrylic acid graftpolymers wherein the carboxyl groups are partially neutralized withsodium hydroxide or potassium hydroxide solution.

For aesthetic reasons and from environmental aspects, there is anincreasing tendency of designing sanitary articles such as diapers forbabies, incontinence articles and liners increasingly smaller andthinner. In order to ensure a constant retention capability of thesanitary articles, the above requirement can only be realized byreducing the percentage of large-volume fluff. As a result, thesuperabsorber also has to assume functions with respect to conveyanceand distribution of liquid, which may be summarized as permeabilityproperties.

Permeability in the case of a superabsorber material is understood to bethe ability of conveying added liquids and distributing them in athree-dimensional fashion in its swollen state. In a swollensuperabsorber gel, this process takes place via capillary conveyancethrough interstices between the gel particles. The actual conveyance ofliquid through swollen superabsorber particles complies with the laws ofdiffusion and is an exceedingly slow process which, in the servicecondition of the sanitary article, does not play any role in thedistribution of liquid. In superabsorber materials incapable ofaccomplishing capillary conveyance due to lacking gel stability,separation of the particles from each other has been ensured byembedding these materials in a fiber matrix, thereby avoiding the gelblocking phenomenon. In new generation diaper constructions, theabsorber layer has only minor amounts of fiber material to support theconveyance of liquid, or none at all. Accordingly, the superabsorbersused therein must have sufficiently high stability in their swollenstate, so that the swollen gel still has a sufficient quantity ofcapillary space, through which conveyance of liquid is possible.

In one aspect, in order to obtain superabsorber materials having highgel strength, the polymer crosslinking level could be increased, whichwould inevitably result in a loss of swelling capacity and retentioncapability. Indeed, an optimized combination of various crosslinkers andcomonomers as described in the patent specification DE 196 46 484 isable to improve the permeability properties, but not to such a levelthat incorporation in a diaper construction of a layer optionallyconsisting of superabsorbers only would be possible.

Furthermore, methods of surface secondary crosslinking of the polymerparticles may be used. During the so-called secondary crosslinking, thecarboxyl groups of the polymer molecules at the surface of thesuperabsorber particles are reacted with various secondary crosslinkingagents capable of reacting with at least two of the carboxyl groups nearthe surface. In addition to increasing the gel strength, the capabilityof absorbing liquids under pressure is highly improved in particular,because the well-known phenomenon of gel blocking is suppressed, whereslightly swollen polymer particles adhere to each other, therebypreventing further absorption of liquid.

The surface treatment of liquid-absorbing resins is already well-known.To improve the dispersibility, ionic complexing of the carboxyl groupsnear the surface using polyvalent metal cations has been suggested inU.S. Pat. No. 4,043,952. This treatment is effected using salts ofmultivalent metals dispersed in organic solvents (alcohols and otherorganic solvents) optionally containing water.

A secondary treatment of superabsorber polymers using reactive,surface-crosslinked compounds (alkylene carbonates) to increase theliquid absorption capability under pressure has been described inDE-A-40 20 780.

The EP 0,233,067 describes water-absorbing resins crosslinked at theirsurface, obtained by reacting a superabsorbent polymer powder with analuminum compound. A mixture of water and diols is used as treatmentsolution, which is intended to render the use of lower alcohols assolvents unnecessary. Preferably, 100 parts of crosslinker solution isapplied on 100 to 300 parts of absorber. According to the examples, thereaction with the aluminum component takes place at room temperature.The diols (e.g., polyethylene glycol 400 and 2000, 1,3-butanediol or1,5-pentanediol) added to the water reaction medium serve to preventaggregation of the superabsorber during the treatment with such largeamounts of aqueous treatment solution used therein. The solvent isremoved in a subsequent drying operation at 100° C. The polymers thustreated have an insufficient level of properties, with improvement ofthe absorption capability under pressure not being achieved.Furthermore, a treatment using large amounts of treatment solution isnot economically feasible in modern, continuously operating processes.

WO 96/05234 describes a process for the production of superabsorbingpolymers, according to which a crosslinked layer is formed at thesurface of the absorber particles containing at least 10 wt.-% of waterby reacting a reactive, hydrophilic polymer or a reactive organometalliccompound with an at least bifunctional crosslinker below 100° C. Thepolymer products are said to have a well-balanced correlation ofabsorption, gel strength and permeability, the measured values havingbeen determined according to extremely poor criteria of evaluation.Thus, for example, the absorption and permeability have been determinedwithout any pressure load. One drawback in this well-known process isthe use of solvents and toxically critical crosslinking reagents such aspolyimines, alkoxylated silicone or titanium compounds, and epoxideswhich are mentioned as being preferred.

An improvement in the properties of permeability and liquid conveyanceis achieved in WO 95/22356 and WO 97/12575 by appropriately treatingcommercially available superabsorber products with aminopolymers inorganic solvents. In addition to using toxicologically criticalpolyamines and polyimines, a serious drawback of the process describedtherein is the use of large amounts of organic solvents required in thetreatment of the polymers. Industrial production is excluded by thesafety aspect and cost associated therewith. In addition to thetoxicological risk of these treatment agents, their tendency todecompose under the high temperatures of secondary crosslinking mustalso be taken into account which, among other things, can be seen in ayellow discoloration of the absorber particles.

The state of the art as described above does not provide any indicationthat a dramatic augmentation in the permeability properties is alsopossible in this secondary crosslinking stage, while retaining highretention capacity and capability of absorbing liquids under pressure.

It was therefore the object of the present invention to providesuperabsorbing polymers which, as a combination of properties, not onlyhave high absorbing capacity under pressure but also the normallycontrary properties of high retention capability and good permeabilityin combination, i.e., a level of combined properties where, in additionto a retention value of at least ≧25 g/g, an SFC value of at least30×10⁻⁷, preferably at least 50×10⁻⁷ cm³·s/g is present. In particular,it was the object to provide superabsorbing polymers which particularlywould be suitable for use in very thin diaper constructions having avery high percentage of superabsorber. In particular, polymers havingretention values of ≧25 g/g and permeability values SFC of >70×10⁻⁷cm³·s/g are required in this case.

It was another object of the invention to find production processes forthese superabsorbing polymers that would be easy, economically andsafely feasible, provide constant product quality and wherein, inparticular, low amounts of solvent are used, and organic solvents areavoided as far as possible. In addition, these processes should befeasible without the use of toxicologically critical substances.

The object of the invention is accomplished by providing a powderedpolymer product which has been subjected to secondary crosslinking atits surface, and which absorbs water, aqueous or serous fluids, as wellas blood, and is constituted of

a) 55-99.9 wt.-%, preferably 70-90 wt.-% of polymerized, ethylenicallyunsaturated monomers which contain acid groups and are neutralized to atleast 25 mole-%,

b) 0-40 wt.-%, preferably 0-20 wt.-% of polymerized, ethylenicallyunsaturated monomers copolymerizable with a),

c) 0.1-5.0 wt.-%, preferably 0.1-3 wt.-% of one or more polymerizedcrosslinking agents,

d) 0-30 wt.-%, preferably 0-5 wt.-% of a water-soluble polymer, the sumof the weight amounts a) through d) always being 100 wt.-% ,characterized in that the polymer product has been coated with

e) 0.01-5 wt.-%, relative to the polymer product, of at least one polyolas surface secondary crosslinking agent in the form of an aqueoussolution, and

f) 0.001-1.0 wt.-%, relative to the polymer product, of a cation in theform of an aqueous solution, and heated to a secondary crosslinkingtemperature of from 150 to 300° C.

Surprisingly, a superabsorber resin with significantly improvedpermeability properties and high retention capability is obtained bycoating a particulate absorber resin with an aqueous solution of apolyol which has reacted with molecular groups near the surface,preferably with carboxyl groups, in the presence of a cation of a saltcomponent with heating at 150 to 300° C.

Quite unexpectedly, the aqueous solution of the inventive combination ofsecondary crosslinker components provides the desired result, namely,superabsorber resins having high retention capability even underpressure and, at the same time, excellent permeability properties.Successive, separate use of an aqueous solution of the organic secondarycrosslinking agent and an aqueous solution of the salt with heating ineach case does not result in comparably good product characteristics.

The sole use of a polyol as organic secondary crosslinking agent inaqueous solution results in products having high retention capacity,high gel strength and high absorption capability under pressure.However, a significant increase of permeability in the swollen state canonly be achieved by a correspondingly higher crosslinking level of thepolymers during polymerization, or by more intensive secondarycrosslinking (increased amounts of secondary crosslinking agent or moresevere conditions) and an associated loss of retention capacity.

Likewise, sole secondary crosslinking using high positive charge densitycations will not result in polymer products having the desiredcombination of properties. In particular, satisfactory values of liquidabsorption under pressure and good permeability properties cannot beachieved. As a consequence, treating superabsorber polymers withmultivalent cations only can only increase the rate of liquidabsorption. An improvement of pressure stability or even liquidconveyance properties in the swollen state is not achieved.

According to the invention, polyols reacting with the surface COOHgroups of the polymer product are used as organic secondary crosslinkercomponent e).

Preferably, aliphatic polyhydroxy compounds such as C₂-C₈ alkylenediols,e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, dianhydrosorbitol, C₂-C₈ alkylenetriols such asglycerol, trimethylolpropane, higher-functional hydroxy compounds suchas pentaerythritol and sugar alcohols, e.g. sorbitol, as well as di- andpolyalkylene glycols such as diethylene glycol, di-propylene glycol,triethylene glycol, tetraethylene glycol, tetrapropylene glycol,polyethylene glycol, polypropylene glycol, polyglycols based on 2 ormore different monomer units, e.g. a polyglycol of ethylene oxide andpropylene oxide units, are used as polyols. The organic secondarycrosslinker component or the mixtures thereof are employed in amounts offrom 0.01 to 5 wt.-%, preferably from 0.1 to 2.5 wt.-%, and morepreferably from 0.5 to 1.5 wt.-%, relative to the polymer product to becrosslinked.

According to the invention, aqueous solutions of salts are preferablyused as component f) to crosslink the carboxylate groups near thesurface, the anions of which are chloride, bromide, sulfate, carbonate,nitrate, phosphate, or organic anions such as acetate and lactate. Thecations preferably are derived from uni- or multivalent cations, theunivalent ones preferably from alkali metals such as potassium, sodium,lithium, with lithium being preferred. Bivalent cations used accordingto the invention are derived from zinc, beryllium, alkaline earth metalssuch as magnesium, calcium, strontium, with magnesium being preferred.Other examples of polyvalent cations which may be used according to theinvention are cations of aluminum, iron, chromium, manganese, titanium,zirconium, and other transition metals, as well as double salts of thesecations, or mixtures of the above-mentioned salts. It is preferred touse aluminum salts and alums and their various hydrates, such asAlCl₃×6H₂O, NaAl(SO₄)₂×12H₂O, KAl(SO₄)₂×12H₂O, or Al₂(SO₄)₃×14-18H₂O. Itis particularly preferred to use Al₂(SO₄)₃ and the hydrates thereof.Calculated relative to the cation, the salt component is employed inamounts of from 0.001 to 1.0 wt.-%, preferably 0.005 to 0.5 wt.-%, andmore preferably 0.01 to 0.2 wt.-%, relative to the polymer product.

The water-absorbing polymer product to be surface-crosslinked isobtained by polymerizing a) 55-99.9 wt.-% of a monounsaturated monomerhaving acid groups, where monomers containing carboxyl groups arepreferred, e.g., acrylic acid, methacrylic acid,2-acrylamido-2-methylpropanesulfonic acid, or mixtures of thesemonomers. Preferably, at least 50% and more preferably at least 75% ofthe acid groups are carboxyl groups. The acid groups are neutralized toat least 25 mole-%, i.e., they are present as sodium, potassium orammonium salts. The degree of neutralization preferably is at least 50mole-%. Particularly preferred is a polymer product obtained bypolymerization, in the presence of crosslinkers, of acrylic acid ormethacrylic acid, the carboxyl groups of which have been neutralized to50-80 mole-%.

From 0 to 40 wt.-% of ethylenically unsaturated monomers copolymerizablewith a), such as acrylamide, methacrylamide, hydroxyethyl acrylate,dimethylaminoalkyl (meth)acrylate, dimethylaminopropylacrylamide, oracrylamidopropyltrimethylammonium chloride may be used as other monomersb) in the production of the absorbent polymer products. Monomers inexcess of 40 wt.-% might deteriorate the swelling capability of thepolymer products.

All those compounds bearing at least two ethylenically unsaturateddouble bonds or one ethylenically unsaturated double bond and onefunctional group reactive towards the acid groups of the monomers a) ormultiple functional groups reactive towards acid groups may be used ascrosslinker component c) present during the polymerization of a) and b).As examples may be mentioned: aliphatic amides such asmethylenebisacryl- or -methacrylamide, or ethylenebisacrylamide, andalso, aliphatic esters of polyols or alkoxylated polyols withethylenically unsaturated acids, such as di(meth)acrylates ortri(meth)acrylates of butanediol or ethylene glycol, polyglycols,trimethylolpropane, di- and triacrylate esters of trimethylolpropanepreferably alkoxylated with from 1 to 30 mol of alkylene oxide,preferably ethoxylated, acrylate and methacrylate esters of glycerol andpentaerythritol and of glycerol and pentaerythritol preferablyethoxylated with from 1 to 30 mol of ethylene oxide, and also, allylcompounds such as allyl (meth)acrylate, alkoxylated allyl (meth)acrylatereacted preferably with from 1 to 30 mol of ethylene oxide, triallylcyanurate, triallyl isocyanurate, maleic acid diallyl ester, polyallylesters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine,allyl esters of phosphoric acid or phosphorous acid, and also,crosslinkable monomers such as N-methylol compounds of unsaturatedamides like methacrylamide or acrylamide and the ethers derivedtherefrom. Mixtures of the above-mentioned crosslinkers may also beused. The percentage of crosslinking comonomers is from 0.1 to 5 wt.-%,preferably from 0.01 to 3.0 wt.-%, relative to the total amount ofmonomers.

From 0 to 30 wt.-% of water-soluble polymer products, such as partiallyor completely saponified poly(vinyl alcohol), polyvinylpyrrolidone,starch or starch derivatives, polyglycols, or poly(acrylic acids) may beincluded as watersoluble polymers d) in the absorbent polymer productsaccording to the invention, preferably incorporated by polymerization.The molecular weight of these polymers is not critical as long as theyare soluble in water. Preferred water-soluble polymers are starch andpoly(vinyl alcohol). The preferred content of these water-solublepolymers in the absorbent polymer product of the invention is around 1-5wt.-%, preferably 0-5 wt.-%, relative to the total amount of componentsa) through d). The water-soluble polymers, preferably synthetic onessuch as poly(vinyl alcohol), may also serve as grafting basis for themonomers to be polymerized.

Conventional initiators such as azo or peroxo compounds, redox systemsor UV initiators (sensitizers) are used to initiate the free-radicalpolymerization.

Preferably, the polymer products of the invention are produced accordingto two methods:

According to the first method, the partially neutralized monomer a),preferably acrylic acid, is converted to a gel by means of free-radicalpolymerization in aqueous solution and in the presence of crosslinkersand optionally other components, which gel is subsequently crushed,dried, milled, and screened to the desired particle size. Such asolution polymerization may be conducted in a continuous or batchwisefashion. The state of the art includes a broad spectrum of possiblevariants with respect to concentration conditions, temperatures, typeand amount of initiators. Typical processes have been described in thepublications U.S. Pat. No. 4,286,082, DE 27 06 135, and U.S. Pat. No.4,076,663, the disclosures of which are hereby incorporated byreference.

Inverse suspension and emulsion polymerization may also be used toproduce the products of the invention. According to these processes, anaqueous, partially neutralized solution of the monomers a), preferablyacrylic acid, is dispersed in a hydrophobic organic solvent usingprotective colloids and/or emulsifiers, and the polymerization isinitiated using free-radical initiators. The crosslinkers are eitherdissolved in the monomer solution and metered together with same oradded separately and optionally during the polymerization. Optionally, awater-soluble polymer d) as a grafting basis is added via the monomersolution or by directly placing in the oil phase. Subsequently, thewater is removed azeotropically from the mixture, and the polymerproduct is filtrated and optionally dried. Crosslinking may be effectedby incorporating a polyfunctional crosslinker via polymerization, whichis dissolved in the monomer solution, and/or by reacting suitablecrosslinking agents with functional groups of the polymer during thepolymerization steps. For example, these processes have been describedin U.S. Pat. No. 4,340,706, DE 37 13 601, DE 28 40 010, and WO 96/05234,the disclosures of which are hereby incorporated by reference.

The polymer gel is dried to a water content of 0.5-25 wt.-%, preferably1-10 wt.-%, and more preferably 1-8 wt.-%, at temperatures normallyranging from 100-200° C.

There are no particular limitations regarding the particle shape of theabsorbent polymer products according to the invention. The polymerproduct may be in the form of beads obtained by inverse suspensionpolymerization, or in the form of irregularly shaped particles derivedfrom drying and powdering the gel mass from solution polymerization. Theparticle size normally is below 3000 μm, preferably between 20 and 2000μm, and more preferably between 150 and 850 μm.

The secondary crosslinker components according to the invention areapplied in the form of their aqueous solutions. Suitable solvents arewater and optionally, polar watermiscible organic solvents such asacetone, methanol, ethanol, or 2-propanol, or mixtures thereof. Withrespect to the solvent component, the term “aqueous solution” in themeaning of the invention indicates that other organic solvents may alsobe included in addition to water. The concentration of each secondarycrosslinker component in the aqueous solvent may vary within widelimits, ranging from 1-80 wt.-%, preferably from 5 to 65 wt.-%, and morepreferably from 10 to 40 wt.-%. The preferred solvent for the polyols assecondary crosslinking agents and for the salt component is water whichis used in an amount of from 0.5 to 10 wt.-%, preferably from 0.75 to 5wt.-%, and more preferably from 1.0 to 4 wt.-%, relative to the polymerproduct.

If the polyol and the salt component are present in an aqueous solution,the soluble quantities of both components may be limited by salting-outeffects and have to be adapted in accordance with the composition.Because the amount of organic solvents must be held as low as possiblefor safety reasons in order to avoid explosions, a stable mixed phase ofwater/organic solvent/organic secondary crosslinker compound/saltcomponent cannot be achieved at any concentration of compound. Forexample, a preferred solution consists of 1.5-3 parts by weight ofwater, 0.5-1 parts by weight of polyol component, and 0.4-0.6 parts byweight of an inorganic salt. Conventionally, the total amount of solventemployed ranges from 0.5-12 wt.-%, preferably from 1 to 7 wt.-%, andmore preferably from 1 to 5 wt.-%, relative to the polymer product.

In addition to water and the above-mentioned organic solvents, othersolubilizers such as inorganic or organic acids or complexing agents mayalso be used in order to reduce the amounts of liquid applied on thepolymer product.

Depending on the solubility of both components e) and f), the solutionis heated to 20-100° C., preferably 20-60° C. prior to application onthe polymer product. Separate, yet simultaneous metering of a solutionof the polyol and a solution of the salt component is also possible ifhomogeneous distribution of both components on the polymer product isensured and the material is subjected to a thermal subsequent treatment.Preferably, one single solution is applied on the polymer product,wherein both components are dissolved.

The secondary crosslinker solution should be mixed thoroughly with thepolymer particles. Suitable mixing units for applying the secondarycrosslinker solution are, e.g., Patterson-Kelley mixers, DRAISturbulence mixers, Lödige mixers, Ruberg mixers, screw mixers, panmixers, and fluid-bed mixers, as well as continuously operated verticalmixers wherein the polymer powder is mixed at a rapid frequency usingrotating knives (Schugi mixer). It is also possible to coat the polymerproduct in the course of a particular processing step during theproduction of the polymer product. To this end, the process of inversesuspension polymerization is particularly suited.

Once the secondary crosslinker solution has been mixed with the polymerparticles, the secondary crosslinking reaction preferably is performedat temperatures ranging from 150° C. to 300° C., preferably from >150°C. to 250° C., and more preferably from 160° C. to 210° C. The optimumtime period for additional heating can easily be determined for eachsingle type of crosslinker in just a few tests and is limited by thatpoint where the desired pattern of properties of the superabsorber isdestroyed as a result of heat damage. The thermal treatment may becarried out in usual dryers or ovens; as examples may be mentioned:rotary-tube dryers, fluid-bed dryers, disc dryers, blade dryers, orinfrared dryers.

The polymers of the invention can be produced on an industrial scaleaccording to well-known processes in a continuous or batchwise fashion.

The polymer products according to the invention may be used in a widefield of applications. If used in liners, diapers or in wound dressings,for example, they have the property of rapidly absorbing large amountsof menstruation blood, urine, or other body fluids. Because the agentsaccording to the invention retain absorbed liquids even under pressureand, in addition, are capable of distributing additional liquid withinthe construction in their swollen state, it is particularly preferred touse them at higher concentrations, relative to the hydrophilic fibrousmaterial such as fluff, than has been possible so far. They are suitablefor use as homogeneous superabsorber layers, with zero amounts of fluffwithin the diaper construction, thereby enabling particularly thindiapers. Furthermore, the polymers are suitable for use in hygienearticles (incontinence articles) for adults.

The polymers according to the invention are also used in absorberarticles suitable for most various purposes, e.g. by mixing with paper,fluff or synthetic fibers, or by dispersing the superabsorber betweensubstrates made of paper, fluff or nonwoven textiles, or by processinginto support materials to form a web. In addition, the polymers of theinvention are used in all those cases where aqueous liquids have to beabsorbed, e.g., in cable sheathings, in food packagings, in theagricultural field in plant breeding, as water reservoir, and as avehicle for active substances involving the function of delayed releaseinto the environment.

Surprisingly, the superabsorbers according to the invention exhibit asignificant improvement in permeability, i.e., an improvement in liquidconveyance in the swollen state. Polymer products are obtained havingpermeability values (SFC) of up to 70×10⁻⁷ cm³·s/g at a retention (TB)of at least 27 g/g, and preferably such polymers having SFC values offrom >70×10⁻⁷ to ≧150×10⁻⁷ cm³·s/g at a retention (TB) of at least 25g/g. In addition to these excellent SFC and retention values, thepolymers of the invention exhibit measured values of liquid absorptionunder pressure (AAP_(0.7)) of at least 18 g/g.

The products of the invention having this outstanding combination ofproperties comprising very high SFC values, high retention and highabsorption under pressure can be produced without using toxicologicallycritical substances.

As can be seen from the following Examples, the secondary crosslinkingaccording to the invention is applicable to a variety of polymerproducts having various chemical structures. Thus, it is no longernecessary to fall back on special crosslinker combinations, comonomersor expensive secondary treatment procedures at such an early stage asduring the production of the polymer products in order to achieve an atleast slightly increased permeability.

Test Methods

In order to characterize the absorbent polymer products of theinvention, the retention (TB), absorption under pressure (AAP) andpermeability for a 0.9% saline solution in the swollen state (SFC) aredetermined.

a) The retention is determined according to the tea bag method and isgiven as mean value of three measurements. About 200 mg of polymerproduct is welded in a tea bag and immersed in a 0.9% NaCl solution for30 minutes. The tea bag is subsequently centrifuged in, a centrifuge (23cm in diameter, 1,400 rpm) for 3 minutes and weighed. A tea bag havingno water-absorbing polymer is run as a blank.${Retention} = {\frac{{{Final}\quad {weight}} - {Blank}}{{Initial}\quad {weight}}\quad \text{[g/g]}}$

b) Liquid absorption under pressure (AAP test according to EP 0,339,461)

The absorption under pressure (pressure load 50 g/cm²) is determinedaccording to the method described in EP 0,339,461, page 7. About 0.9 gof superabsorber is weighed in a cylinder having a screen bottom. Theuniformly spread superabsorber layer is loaded with a piston exerting apressure of 50 g/cm². The previously weighed cylinder then is placed ona glass filter plate situated in a tray containing a 0.9% NaCl solution,the liquid level of which precisely corresponds to the height of thefilter plate. After allowing the cylinder unit to absorb 0.9% NaClsolution for 1 hour, it is reweighed, and the AAP is calculated asfollows:

AAP=Final weight (cylinder unit+superabsorber)−Initial weight (cylinderunit+soaked superabsorber)/Initial weight of superabsorber

c) Permeability in the swollen state (SFC Test according to WO 95/22356)

0.9 g of superabsorber material is weighed in a cylinder having a screenbottom and spread carefully over the screen surface. The superabsorbermaterial is allowed to swell in JAYCO synthetic urine [composition: 2.0g of potassium chloride, 2.0 g sodium sulfate, 0.85 g of ammoniumdihydrogen phosphate, 0.15 g of ammonium hydrogen phosphate, 0.19 g ofcalcium chloride, 0.23 g of magnesium chloride as anhydrous saltsdissolved in 1 l of distilled water] for 1 hour against a pressure of 20g/cm². After detecting the swelling height of the superabsorber, a 0.118M NaCl solution is allowed to flow from a levelled reservoir vesselthrough the swollen gel layer at a constant hydrostatic pressure. Duringmeasurement, the swollen gel layer is covered with a special screencylinder which ensures uniform distribution of the 0.118 M NaCl solutionabove the gel and constant conditions (measuring temperature 20-25° C.)during measurement with respect to the gel bed structure. The pressureacting on the swollen superabsorber still is 20 g/cm². Using a computerand a scale, the amount of liquid passing through the gel layer as afunction of time is detected at intervals of 20 seconds within a timeperiod of 10 minutes. The flow rate g/s through the swollen gel layer isdetermined using regression analysis with slope extrapolation anddetermination of the center to time t=0 of the flow rate over theminutes 2-10. The SFC value (K) is calculated as follows:$K = {\frac{{F_{s}\left( {t = 0} \right)} \times L_{0}}{r \times A \times \Delta \quad P} = \frac{{F_{s}\left( {t = 0} \right)} \times L_{0}}{139506}}$

wherein

F_(s) (t = 0) flow rate in g/s L₀ thickness of gel layer in cm r densityof NaCl solution (1.003 g/cm³) A area of upper side of gel layer in themeasuring cylinder (28.27 cm²) ΔP hydrostatic pressure resting on gellayer (4920 dyne/cm²) K SFC value [cm³ · s · g⁻¹]

Formal addition of the figures from tea bag retention and the SFC valueillustrates the abrupt increase of this combination of properties in thepolymer products of the invention as compared to non-treatedsuperabsorber powder or to products subjected to surface secondarycrosslinking according to well-known methods. In the products accordingto the invention, said figure is not achieved by a high contribution ofone of these two values (e.g., a high TB retention value and a low SFCvalue or vice versa).

EXAMPLES

In the Examples and Comparative Examples, each powder intended forsurface secondary crosslinking was screened to a particle size of from150 μm to 850 μm.

Example 1

A polyacrylic acid (powder A) was obtained by a production processwherein the content of acrylic acid, which had been neutralized to 70%,in the aqueous monomer solution was 26 wt.-%, and was crosslinked using0.7 wt.-%, relative to acrylic acid, of a mixture of the crosslinkerstriallylamine and polyethylene glycol diacrylate. Following drying andmilling of the polymer product, screening to a particle size of 150-850μm was effected, and 100 g of the powder a) was mixed with a solution of1 g of ethylene glycol, 2.5 g of water and 0.5 g of aluminum sulfate14-hydrate with vigorous stirring and subsequently heated for 60 minutesin an oven heated to 175° C.

TB AAP_(0.7) SFC Product [g/g] [g/g] [10⁻⁷ cm³ · s/g] TB + SFC Powder A33.5 Example 1 28.5 25 65 93.5

Comparative Examples 1-8

100 g of the polymer powder A or powder B, Favor® SXM 6860 (seeComparative Example 13), was coated with the solutions specified belowwith thorough mixing and subsequently dried (60° C., 60 minutes); cf.,EP 0,233,067.

Solution A: 25 g of a solution of polyethylene glycol 400 (8 parts),AlCl₃×6H₂O (20 parts), and water (72 parts). Solution B: 25 g of asolution of polyethylene glycol 400 (8 parts), Al₂(SO₄)₃×14H₂O (20parts), and water (72 parts). Solution C: 25 g of a solution of1,3-butanediol (8 parts), AlCl₃×6H₂O (20 parts), and water (72 parts).Solution D: 25 g of a solution of 1,3-butanediol (8 parts),Al₂(SO₄)₃×14H₂O (20 parts), and water (72 parts).

Comp. Ex. TB AAP_(0.7) SFC (Powder) Solution [g/g] [g/g] [10⁻⁷ cm³ ·s/g] TB + SFC 1 (A) A 26 15  1 27 2 (B) A 27 19 10 37 3 (A) B 26 14  733 4 (B) B 27 19 13 40 5 (A) C 26 15  7 33 6 (B) C 27 17 15 42 7 (A) D26 15  8 34 8 (B) D 26 18 20 46

Comparative Examples 9-12

100 g of the polymer powder A or powder B, Favor® SXM 6860 (seeComparative Example 13), was coated with the solutions specified belowwith thorough mixing and subsequently dried (100° C., 90 minutes); cf.,EP 0,233,067.

Solution E: 50 g of a solution of 1,3-butanediol (15 parts), AlCl₃×6H₂O(31 parts), and water (85 parts). Solution F: 50 g of a solution of1,3-butanediol (15 parts), Al₂(SO₄)₃×14H₂O (31 parts), and water (85parts).

Comp. Ex. TB AAP_(0.7) SFC (Powder) Solution [g/g] [g/g] [10⁻⁷ cm³ ·s/g] TB + SFC  9 (A) E 25 16  2 27 10 (B) E 17 16 10 27 11 (A) F 24 1718 42 12 (B) F 24 15 42 66

Comparative Example 13

100 g of Favor® SXM 6860 (commercial product by the company StockhausenGmbH & Co., secondary surface-crosslinked polyacrylate) was mixed with asolution of 2.5 g of water and 0.5 g of aluminum sulfate 14-hydrate withvigorous stirring and subsequently heated for 30 minutes in an ovenheated to 180° C.

TB AAP_(0.7) SFC Product [g/g] [g/g] [10⁻⁷ cm³ · s/g] TB + SFC Powder B31.5 25.5  5 36.5 Comparative Example 13 27   21.5 15 43  

Example 2

A powdered polyacrylic acid (powder C, 100 g) crosslinked with 0.8 wt.-%of polyethylene glycol diacrylate, relative to acrylic acid, and presentto 70 mole-% neutralized as sodium salt, was screened to 150-850 μmafter drying and milling, mixed with a solution of 1 g of ethyleneglycol, 2.5 g of water and 0.5 g of aluminum sulfate 18-hydrate withvigorous stirring and subsequently heated for 30 minutes in an ovenheated to 180° C.

TB AAP_(0.7) SFC Product [g/g] [g/g] [10⁻⁷ cm³ · s/g] TB + SFC Powder C32.5 10    0 32.5 Example 2 28.5 23.0 50 78.5

Examples 3 and 4

100 g at a time of a powdered crosslinked polyacrylic acid (powder D)present to 70 mole-% as sodium salt was screened to 150-850 μm afterdrying and milling and mixed with solutions having compositions asspecified in the Table below with vigorous stirring and subsequentlyheated in an oven according to the conditions indicated below:

Al₂(SO₄)₃*** Glycol* H₂O TB AAP_(0.7) SFC T/t Product [g] [g] [g] [g/g][g/g] [10⁻⁷ cm³ · s/g] TB + SFC [° C./min] Powder D 31  10  0 31 Example3 0.5 1* 3 26 22.5 95 121 180/30 Comp. Ex. 14 1* 3 26.5 24 42 68.5180/30 Comp. Ex. 15 1* 3 30.5 10  0 30.5 149/60 Example 4 0.5   0.8** 326 23.5 83 109 185/40 Comp. Ex. 16   0.8** 3 26 24 53 79 185/40*Ethylene glycol **Propylene glycol ***Al₂(SO₄)₃ × 18H₂O

The Examples show a significant improvement in the permeability of thepolymer products of the invention in their swollen state, characterizedby the SFC value. Despite high permeability, the other two relevantparameters, i.e., tea bag retention and absorption of liquid underpressure (AAP_(0.7)) are on a high level. It has also been demonstratedthat an appropriate combination of properties comprising high retentioncapability, good absorption of liquid under pressure and highpermeability in the swollen state can be achieved by a treatment using acombination of polyol and an inorganic salt component with heating ofthe coated polymer product to at least 150° C. The sole use of saltcomponent (Comparative Example 13) or crosslinking at a reactiontemperature below the one according to the invention (ComparativeExamples 1-12 and 14) does not result in the desired pattern ofproperties. The products obtained according to the Comparative Examplesdo not even nearly result in superabsorbers that would be comparable tothe products of the invention. Moreover, when coating the polymerproducts using large amounts of aqueous solutions or organic solvents,serious problems will arise with respect to the feasibility of theprocess (massive aggregation of material, and large amounts of organicvapors to be removed).

What is claimed is:
 1. A powdered polymer product which has beensubjected to secondary crosslinking at its surface, comprising a polymercomprising a) 55-99.9 wt.-% of polymerized, ethylenically unsaturatedmonomers which contain acid groups and are neutralized to at least 25mole-%, b) 0-40 wt.-% of polymerized, ethylenically unsaturated monomerscopolymerizable with a), c) 0.1-5.0 wt.-% of one or more polymerizedcrosslinkers, d) 0-30 wt.-% of a water-soluble polymer, the sum of theweight amounts a) through d) being 100 wt.-%, wherein the polymer hasbeen treated with e) 0.01-5 wt.-%, relative to the polymer product, ofat least one polyol as surface secondary crosslinking agent in anaqueous solution, and f) 0.001-1.0 wt.-%, relative to the polymerproduct, of a cation in the form of an aqueous solution, and heated to asecondary crosslinking temperature of from 150 to 300° C., wherein thepowdered polymer product has a permeability of 70×10⁻⁷ cm³·s/g orgreater.
 2. The polymer product according to claim 1, wherein componente) is employed with 0.1 to 2.5 wt.-%, and component f) with 0.005 to 0.5wt.-%.
 3. The polymer product according to claim 1, wherein water issolely employed as solvent for components e) and f).
 4. The polymerproduct according to claim 1, wherein the components e) and f) areemployed together in an aqueous solution.
 5. The polymer productaccording to claim 1, wherein the total amount of water in the aqueoussolutions added separately or together is 0.5 to 10 wt.-%, relative tothe polymer product.
 6. The polymer product according to claim 1,wherein the cation is selected from the group consisting of an alkali oralkaline earth metal, zinc, iron, aluminum, titanium, another transitionmetal, a double salt of two different cations, and a mixture of salts.7. The polymer product according to claim 1, wherein C₂-C₈alkylenediols, C₂-C₈ alkylenetriols, higher-functional hydroxy compoundsand/or di- and polyalkylene glycols are employed as polyols.
 8. Thepolymer product according to claim 1, wherein the secondary crosslinkingtemperature is from 150° C. to 250° C.
 9. The polymer product accordingto claim 1, wherein at least 50% of the acid groups of the monomer unitsa) are carboxyl groups.
 10. The polymer product according to claim 1,wherein the monomer units a) are derived from acrylic acid and/ormethacrylic acid.
 11. The polymer product according to claim 1, whereinstarch and/or poly(vinyl alcohol) or derivatives thereof are used ascomponent d).
 12. The polymer product according to claim 1, wherein thepolymer product has a retention (TB) of at least 27 g/g at apermeability (SFC) of 70×10⁻⁷ cm³·s/g.
 13. The polymer product accordingto claim 1, wherein the polymer product has a retention (TB) of at least25 g/g at a permeability (SFC) of from 70×10⁻⁷ to 150×10⁻⁷ cm³·s/g. 14.The polymer product according to claim 12, wherein the polymer producthas a liquid absorption under pressure (AAP_(0.7)) of at least 18 g/g.15. A process for producing the absorbent polymer product according toclaim 1, wherein a mixture of a) 55-99.9 wt.-% of ethylenicallyunsaturated monomers which contain acid groups and are neutralized to atleast 25 mole-%, b) 0-40 wt.-% of ethylenically unsaturated monomerscopolymerizable with a), c) 0.1-5.0 wt.-% of one or more crosslinkercompounds, d) 0-30 wt.-% of a water-soluble polymer, the sum ofcomponents a) through d) being 100 wt.-%, is subjected to free-radicalpolymerization, optionally crushed, dried, powdered, screened, and thatthe polymer powder is treated with e) 0.01-5 wt.-%, relative to thepolymer product, of at least one polyol as surface secondarycrosslinking agent in an aqueous solution, and f) 0.001-1.0 wt.-%,relative to the polymer product, of a cation in an aqueous solution,wherein intense mixing of the aqueous solutions of components e) and f),which are present together or separately, with the polymer powder iseffected, and thermal secondary crosslinking of the polymer powder iseffected by subsequent heating to 150-300° C.
 16. The process accordingto claim 15, wherein the polymer powder employed has a moisture contentof from 0.5 to 25 wt.-%.
 17. The process according to claim 15, whereinthe polymer powder employed has a particle size of <3000 μm.
 18. Theprocess according to claim 15, wherein the aqueous solutions ofcomponents e) and f) are heated to 20° C.-100° C.
 19. The processaccording to claim 15, wherein heating to temperatures of from 150° C.to 250° C. is effected.
 20. A method comprising absorbing water oraqueous liquids wherein a polymer product according to claim 15 ispresent as an absorbent in foamed and non-foamed sheet materials, inpackaging materials, in constructions for plant breeding, as soilimprovers, or as vehicles for active substances.
 21. A method comprisingpreparing an absorbent construction wherein the polymer productsaccording to claim 1 is a predominant to exclusive absorbent in a layerof an absorbing insert.
 22. The polymer product according to claim 1,wherein component e) is employed at 0.5 to 1.5 wt.-%.
 23. The polymerproduct according to claim 1, wherein component f) is employed at 0.01to 0.2 wt.-%.
 24. The polymer product according to claim 1, wherein thetotal amount of water in the aqueous solutions added separately ortogether is 0.75 to 5 wt.-% relative to the polymer product.
 25. Thepolymer product according to claim 1, wherein the total amount of waterin the aqueous solutions added separately or together is from 1.0 to 4wt.-% relative to the polymer product.
 26. The polymer product of claim1 wherein an aluminum salt is used as component f).
 27. The polymerproduct according to claim 1, wherein the secondary crosslinkingtemperature is from 180° C. to 210° C.
 28. The polymer product accordingto claim 1, wherein at least 75% of the acid groups of the monomer unitsa) are carboxyl groups.
 29. The process according to claim 15, whereinthe polymer powder employed has a moisture content from 1 to 10 wt.-%.30. The process according to claim 15, wherein the polymer powderemployed has a moisture content from 1 to 8 wt.-%.
 31. The processaccording to claim 15, wherein the polymer powder employed has aparticle size from 2 to 2000 μm.
 32. The process according to claim 15,wherein the polymer powder employed has a particle size from 150 to 850μm.
 33. The process according to claim 15, wherein the aqueous solutionsof components e) and f) are heated to 20° C.-60° C. prior to use. 34.The process according to claim 15, wherein heating to temperatures offrom 160° C. to 210° C. is effected.
 35. The method of claim 20 whereinthe polymer product is present in a construction for absorbing bodyfluids.
 36. The polymer product according to claim 1, wherein thepolymer product has a permeability (SFC) of from 70×10⁻⁷ to 150×10⁻⁷cm³·s/g.
 37. The polymer product according to claim 1, wherein thepolymer product has a permeability (SFC) of at least 50×10⁻⁷ cm³·s/g.